In recent years, functional materials with novel properties have been intensively studied. In particular, organic polymers combined with metals are expected to provide new optical, electronic, magnetic, catalytic and various other functions. Such organic polymer-metal composite materials are finding many applications in light-emitting devices, energy-converting materials, drug delivery, sensors, high-performance catalysts, solar batteries and other technical fields.
Organic polymers are soft materials that have spaghetti-like molecular structures with an extremely high degree of freedom. Since organic polymers have a distribution of molecular weights, composites of organic polymers and metals are generally provided in the form of mere statistical mixtures. Thus, coordination polymers, organic polymers that can coordinate with metals, are needed to obtain organic polymer-metal composite materials that exhibit novel useful functions. A technique using bipyridyl derivatives as such coordination polymers is known (See, for example, Patent Document 1).
Light modulation devices, display devices and other optical devices using electrochromic materials have also become the subjects of intensive studies, recently. Such electrochromic materials include inorganic materials, such as tungsten oxide, organic materials, such as viologens, and conductive polymer materials.
It is desirable that these electrochromic materials, when used in light modulation devices, display devices and other optical devices, can be readily switched between the colored state and the colorless state. Although inorganic materials and organic materials can achieve favorable colorless state (transparency), their colored state is undesirable. Conductive polymer materials, on the other hand, can achieve desirable colored state, but their colorless state (transparency) is undesirable. This is because the conductivity of the conductive polymer materials results from the n-electron conjugated system. Non-doped conductive polymer materials thus absorb significant amounts of light and exhibit colors such as dark yellow, red, green and deep blue.
An electrochromic display device technology has been developed that uses a conductive polymer material that has overcome the problem of the undesirable colorless state (See, Patent Document 2). The electrochromic display device used in this technology comprises a first transparent electrode, a graft conductive polymer layer arranged in contact with the first transparent electrode, an electrolyte layer arranged in contact with the graft conductive polymer layer, and a second electrode with which the first transparent electrode sandwiches the graft conductive polymer layer and the electrolyte layer. The graft conductive polymer layer is formed of a conductive polymer backbone linked to a conjugated molecular pendant via a metal or a metal ion.
The conjugated molecular pendant causes the conductive polymer material to change its unique state of π-electron bonding or π-π* transition energy. This in turn causes the light absorption band of the conductive polymer material to shift to a shorter or longer wavelength range. As a result, the light absorption in the visible range is decreased to an unnoticeable degree and the conductive polymer material becomes transparent.